java.lang.IllegalStateException

java.lang.IllegalStateException: Yaml is wrong configured: '' is not a valid language. See configuration comments for details.
	at de.leonhard.xcore.data.yaml.language.LanguageManager.getLanguage(LanguageManager.java:30)
	at de.leonhard.xcore.XCoreSpigot.onStart(XCoreSpigot.java:97)
	at de.leonhard.xcore.sapphire.Sapphire$Spigot.onEnable(Sapphire.java:302)
	at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:264)
	at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:337)
	at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:403)
	at org.bukkit.craftbukkit.v1_12_R1.CraftServer.enablePlugin(CraftServer.java:381)
	at org.bukkit.craftbukkit.v1_12_R1.CraftServer.enablePlugins(CraftServer.java:330)
	at net.minecraft.server.v1_12_R1.MinecraftServer.t(MinecraftServer.java:422)
	at net.minecraft.server.v1_12_R1.MinecraftServer.l(MinecraftServer.java:383)
	at net.minecraft.server.v1_12_R1.MinecraftServer.a(MinecraftServer.java:338)
	at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:272)
	at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:545)
	at java.lang.Thread.run(Thread.java:748)